In terms of stuff you can take with you, any sort of resistance tubing with a solid door anchor system is going to be good. I personally like the stuff from lifeline usa since it seems to hold up well over time in my experience.

Add to that list a suspension trainer like a TRX, Jungle Gym XT, Blast Straps, etc and your options are increased by a whole lot. I'd warn that although the TRX is the most portable of the ones I've used, it also requires the most space because of the length of the straps. That might be an advantage if you can sling it over a bar, tree, or power rack, but the Jungle Gym XT can be shortened a good deal almost to the point of doing chins. The downside is that it does not break down to as small a size and it is heavier to carry.

I am not a shill for any of these companies even though it may sound like it a little bit.

Pullups are your friend. When I'm traveling, I can usually find a pullup bar in the hotel gym. A couple times I had to take the lat machine bar and lay it across two machines, but it worked. I've also used children's park equipment for pullups. You would think that would be wierd with kids running around, but it's not because we have fat american kids and they are all at home playing Xbox. Then pushups, squats, and dips.

I also like to do footwork drills. Just push the furnature around till you have a decent space. Then drill away: 7- 3 minute rounds does wonders.

I was really frustrated with this when I went to Greece this past summer.
Seeing as I was trying to travel on the cheap, many of the hotels I stayed in didn't have gyms. In fact, only one did and it was the only time I got a real work-out in.
However, swimming was my saving grace as some of the places had pools, but no gym. Plus there were the beaches..
Daily push- ups become mandatory after a week.

" If one wants to have a friend one must also want to wage war for him: and to wage war one must be capable of being an enemy." - Fr. Nietzsche 'On The Friend' Thus Spake Zarathustra

All the body-weight suggestions above point to an obvious solution: even before you do any travelling, learn to keep yourself in condition in a way that can be done anywhere and does not need a gym. That way--travel or no travel--you will keep yourself in condition. Simple.

Sweet. I was selling stuff last weekend, and I tried to take a break now and then from minding my table to do some decline pushups and fingertip pullups on a doorframe.
Turns out, those get harder when you're wearing a fucking suit of armor.
Anyways, I'm totally going to be adding burpees and box jumps/something like that to the routine.

The fool thinks himself immortal,
If he hold back from battle;
But old age will grant him no truce,
Even if spears spare him.